Todd:
Boys are born with the cells which will make sperm in a very undeveloped
stage. These cells found in the testes are called spermatogonia and they
will eventually produce all the sperm cells the man will have. Around the
time of puberty (http://www.youthshakers.org/sexualhealth/youngmen/), the
spermatogonia will begin to change and start the process of sperm
development called spermatogenesis
(http://www.gpnotebook.co.uk/cache/892665869.htm). This usually begins
sometime around 10-12 years of age and is fully functional by the age of
14-16 years. As the boy continues to mature, the number of sperm cells
increases from few at 10-12 with a full complement of sperm cells by age
16. It is possible for very young boys to father children as the number
of sperm in sufficient at puberty to fertilize oocytes.
I hope this helps!!
David Mallory, PhD
Marshall University
Huntington, WV USA